Metal hydrides are of great interest as energy storage media. Hydrides of magnesium and magnesium alloys are particularly attractive as they combine potentially high hydrogen storage capacities, 7.6 wt % for pure MgH2, with low cost and convenient hydride heats of formation. Practical application is however limited due to poor sorption kinetics. For example, conventional hydrogenation of magnesium requires prolonged treatment at temperatures of 300° C. and above.
Recent studies by Zaluska, Jn. of Alloys and Compounds, 288, (1999), p. 217-225, have shown that the use of high energy ball milling can improve the hydrogen absorption kinetics of magnesium by promoting a nanocrystalline microstructure. Such processing increases the surface area of the metal so that hydride formation is not limited to the surface regions of the metal and also introduces numerous structural defects which facilitate hydrogen penetration. It is important that the milling process is performed under an inert atmosphere, e.g. argon, to prevent oxidation of the magnesium. Absorption kinetics can be improved such that the time for hydrogenation at 300° C. is reduced to a few minutes. Nonetheless, this temperature is still too high for many practical purposes. Other methods of enhancing sorption kinetics have included the use of additives and catalysts. For example, the addition of small amounts of 3d transition metals, such as Ti, V, Mn, Fe or Ni has been reported to allow hydrogen absorption at room temperature and subsequent desorption at 235° C. albeit under reduced pressure.
WO 9623906 describes the use of high energy ball milling to produce nanocrystalline magnesium and magnesium alloy powders with good hydrogen sorption characteristics. Clusters of platinum group metals, Pd, Pt, Ru, Rh, Ir and Os (referred to hereinafter as PGM) may be attached to the surface of the magnesium particles to catalyse the absorption of hydrogen. The PGM is introduced towards the end of the milling process. The materials are described as being able to absorb and desorb hydrogen at room temperature and under both low and high pressure however, the specification only contains examples of experiments conducted at 230° C. and above. The importance of milling under an inert atmosphere to prevent oxidation of the powders is again stressed. The process has two main drawbacks; firstly the PGM are introduced in metallic (elemental) form which, when finely divided, are extremely pyrophoric, and secondly the processed material still has to be charged with hydrogen before it can be used. This requires extra plant in the form of a hydrogenation vessel, and also equipment to transport the material from the milling apparatus and to the point of use whilst guarding against contamination.
Orimo et al., Acta mater. 45, (1997), p. 331-341, describe the milling of Mg2Ni under an atmosphere of hydrogen to produce a nano-structured magnesium-nickel hydride. Care was taken to ensure that no other elements were introduced during processing so avoiding impurity effects on the hydriding and structural properties of the materials.